Method of coating cupreous metal with tin



Patented Feb. 13, 1945 UNITED STATES ENT FIE METHOD OF COATING CUPREOUSMETAL WITH TIN No Drawing. Application March 7, 1941, Serial No. 382,140

2 Claims.

Our invention relates to a method of coating metals and alloys withother metals. More particularly, it relates to a method of coatingmetals and alloys with other metals by an immersion process, without theuse of an electric current from an outside source and without a contactmetal within the bath to be uiltized.

In our United States Patent No. 2,159,510 issued May 23, 1939, wedisclose an immersion process for coating copper or its alloys with tin.The process of the present invention is likewise applicable to coatingcopper and its alloys with tin, although it is also applicable tocoating other metals and alloys with tin or other metals. Although thesodium cyanide-sodium stannite bath and the process disclosed in theUnited States Patent No. 2,159,510 work successfully, we have found thata process employing an immersion coating bath containing thioureapossesses a number of advantages over the process disclosed in saidpatent.

One of the objects of our invention is to produce coatings of tin,antimony, bismuth, silver, lead, molybdenum, and other metals on copper,iron, steel and other metals and alloysby a simple immersion process.

Another object of our invention is to obtain coatings of tin on copperor its alloys or on copper or copper alloy-coated articles by a simpleand relatively inexpensive immersion process wherein the coating may beproduced in a relatively short time.

Another object of our invention is to produce coatings of the typeindicated which will be sumciently thick and will have adequate coveringcharacteristics.

Other objects of our invention will appear from the followingdescription and claims.

The invention will be described first by outlining it as it applies tothe deposition of a coating of tin on copper by immersion; However, itis to be understood that this example is only illustrative and that ourinvention is not limited thereto. Other examples will be given in thedescription following which will show that our invention is alsoapplicable to the deposition of a coating of metals other than tin onmetals other than copper.

We have found that in immersion coating, it is desirable that the metalto be plated be in the form of a complex so that too high aconcentration of free ions does not exist inthe bath. In coating metalswithtin, it is usually necessary to have the tin in the stannouscondition. Thiourea is a strong complex former and also a re-. ducingagent. We have discovered that baths containing thiourea, tin, and anacid produce bright adherent coatings on copper when the latter isimmersed in a solution thereof. We have also discovered that coatingsare effected even it the original bath is made from stannic tin. Whilewe do not wish to be bound by any specific theory,

we are of the opinion that thiourea forms a stable complex with tin and,because of its potency as a reducing agent in acid solutions, part ofthe tin is reduced to the stannous condition in which form it iefiective to make immersion coatings. Also, as is well-known, stannoustin oxidizes readily in solution. We believe that the thiourea reducesstannic tin, formed in this manner, back to the stannous condition.Furthermore, as will be shown later in this description, the bath lifeis much longer in the case of thiourea-containing baths.

It is only necessary to make a solution of a tin salt, thiourea and anacid, and to immerse a piece of copper in the solution. Almostimmediately a coating of tin forms on the copper and the thicknessincreases with increased time of immersion. Coatings are formed atordinary room temperatures, although higher temperatures, even to theboiling point, can be employed.

As specific examples of baths that produced satisfactory coatings of tinon copper, the following are given as illustrative of those tried in theinvestigation leading to this invention.

A bath was made containing 45 grams per liter of thiourea and 5.0 gramsper liter of stannous chloride, SnCl2-2H2O. To this was added sulphuricacid in amounts varying from 1 to grams per liter. Each bath waseffective in producing immersion coatings of tin on copper at roomtemperature.

' In another series the SnClz-ZHzO was kept constant at 5.0 g./l. andthe sulphuric acid at 20 g./l. The thiourea was varied from 1 to 100g./l. Effective coatings were obtained at concentrations of thioureaabove 10 grams per liter. A concentration of 5 g./l. of thiourea wasineffective at room temperature.

In still another series the concentration of thiourea was kept constantat 50 g./l.; the concentration of sulphuric acid constant at 20 g./l.;and the SnCl2-2H2O varied from 0.5 to 100 g./l. All

baths yielded satisfactory coatings, although A hath made of 50 g./l. ofthiourea, 20 g./l. of sulphuric acid and g./l. sodium stannate: i. e.,with tin added in the stannic condition, gave satisfactory coatingseven. at room temperature.

Coatings are deposited rapidly. As an example, a thiourea bathcontaining 5.0 g./l., SnCla-ZHzO deposited tin at room temperature tothe extent of 0.015 gm. per sq. dm. in 5 minutes, 0.029 in 15 minutesand 0.033 in 30 minutes. The deposit in 24 hours was 0.23 g./sq. dm. Therate of deposition varies with the bath composition.

Baths made of thiourea are quite stable. For example, baths containing45 grams of thiourea, 5.00 8J1. SnCh-2H2O and from '5 to 50 8J1. H2804were still effective even after standing open exposed to air for morethan a month.

Coatings may be put on copper-coated articles as well as on the metalitself. For example, a tin immersion coating may be put on coppercladsteel. The copper can be put on the steel by any of the various methods,including electroplating and immersion.

It is obvious, of course, that immersion-coated articles may beheat-treated. For example, copper coated with tin may be heated to forman alloy layer. This unit, if desired, can again be immersed in thetinning bath to give an exterior tin coating.

The advantages of this invention will be obvious to those skilled in theart. The advantages discussed in the United States Patent No. 2,159,-510 for tinned copper are also applicable to this invention. Aparticular and specific advantage of this present invention, however, isthe stability of the baths. Another advantage is that many metals, otherthan tin, can be plated by an immersion process from baths containingthiourea. A further advantage is that tin may be plated from bathsinitially made with stannic tin.

In the foregoing examples, stannous chloride was disclosed as a sourceof tin. Other tin salts,

soluble in the solution used, are

likewise appli= cable.

For example, we may use stannous sulphate or sodium stannite. Likewise,sulphuric thereafter removing said surface from contact with saidsolution, said solution containing, per liter, acid equivalent to thatobtained from i to grams of concentrated sulfuric acid, from 10 to 100grams of thiourea, and dissolved tin equivalent to that obtained by theaddition of from 0.5 to 20 grams of SnClz-2HzO.

2. The method of forming an adherent tin coating on a cupreous surface,said method comprising contacting said cupreous surface with an aqueousacidic solution for a time sufiicient to deposit the desired amount oftin coating, and thereafter removing said surface from contact with saidsolution, said solution containing, per liter, from 1 to 100 grams ofconcentrated sulfuric acid, from 10 to 100 grams of thiourea, anddissolved tin equivalent to that obtained by the addition of from 0.5 to20 grams of SnCl2'2HzO.

- JOHN D. SULLIVAN.

ARNOLD E. PAVLISH.

